1. Field of the Invention
The invention relates to devices for detecting the underinflation of a tire of a motor vehicle while it is traveling, of the type comprising a first electric circuit mounted on the wheel so as to turn with it near a second nonrotating electric circuit mounted near the wheel so that the first circuit induces in the second circuit a magnetic reaction flux established or cancelled by the action of a pressure sensitive device and by the rotation of the wheel.
2. Description of the Prior Art:
Such a device, described in French patent No. 2,459,146, is already known. According to this patent, the first circuit constitutes a resonator unit mounted on the rim of the wheel, while the second circuit is an oscillator unit which is opposite the first when the wheel is in a certain angular position during its rotation. A manometric contact, closed or open in response to the inflation pressure and to the lowering of this pressure, makes it possible to modify the characteristics of the resonator by inserting, for example, a capacitor into the coil with which it is provided when the pressure becomes lower than an acceptable pressure. The oscillator unit then produces a signal which triggers a display or alarm device. This device is sensitive to the adjustment deviations of the resonance frequency and it is difficult to obtain accuracy in the detection. In addition, it has the disadvantage of being placed in a location particularly exposed to the attacks of the environment.
Moreover, in another field, wheel speed sensor devices such as the one described in French patent No. 2,275,775 are known. This latter device comprises a phonic wheel consisting of a crown which itself comprising interruptions and which is mounted to rotate near an electromagnetic detector mounted on a nonrevolving part of the vehicle so that during the rotations of the phonic wheel, the solid parts and the interrupted parts defining teeth pass alternately before the detector. Consequently, the reluctance of the magnetic circuit of the detector varies and a signal formed by succesive pulses is delivered by the coil of the detector, then sent to an electronic device which measures the frequency of it and converts it into the speed of the vehicle. This device provides no indication of the state of inflation of the tire carried by the wheel.